1. Field of the Invention
The invention relates to a network analyzer for analyzing a test object which can be connected thereto.
2. Related Technology
An excitation signal is made available by a network analyzer for analyzing a test object. The excitation signal is fed to said test object via gates thereof. For example, in order to determine an input reflection coefficient and a forward transmission coefficient, and an output reflection coefficient and a reverse transmission coefficient, the excitation signal is fed to the test object, which comprises an input gate and an output gate, via said input gate or said output gate. In order to determine the reflection coefficients and the transmission coefficients, the respective incident and the returning waves of the excitation signal at the input gate and/or the output gate of the test object are to be determined. In order to determine the frequency range at which the excitation signal passes through the test object, and the frequency range at which the excitation signal is reflected at the input gate and/or the output gate of the test object, the network analyzer drives the frequency of the excitation signal through a predetermined frequency range in a frequency sweep.
A network analyzer of this type is known, for example, from the publication DE 102 46 640 A1.
In FIG. 4, the gate parameter “forward transmission coefficient s21” of a test object, which is formed as a band-pass filter, is shown in relation to frequency. The gate parameter s21 describes the relationship between the returning wave b2 at the output gate of the test object, which wave is transmitted through the test object, and the incident wave a1 of the excitation signal at the input gate. The excitation signal is reflected at the input gate of the band-pass filter in both frequency range A and in frequency range C. In frequency range B, the excitation signal is transmitted through the band-pass filter.
In a conventional network analyzer, the transition between the frequency ranges A and C, denoted in the following as the stop band, and the frequency range B, denoted in the following as the pass band, can only be measured comparatively inaccurately. In addition, the frequency range at which the respective transition takes place can only be inaccurately determined. Furthermore, a network analyzer of this type cannot readily analyze the test object over a comparatively large frequency range of from particularly low to particularly high frequencies.
On the one hand, this is due to the fact that the network analyzer sweeps a specific frequency range. However, decoupling a reference signal which corresponds to the excitation signal, and decoupling a measuring signal which corresponds to a signal returning from the test object, on a measuring bridge provided on the network analyzer as a resistive bridge for analyzing the excitation signal which is reflected at the input gate or the output gate of the test object or is transmitted through the test object, is problematic particularly at comparatively high frequencies, due to parasitic elements emerging on the measuring bridge, by means of which disruptive influences on the decoupled signals become noticeable.
On the other hand, this is due to the fact that the coupling paths provided in the measuring bridge formed as a directional coupler are only intended for a specific frequency range. For example, decoupling a reference signal and/or a measuring signal comprising an excitation signal of a comparatively low frequency requires particularly long coupling paths, which can only be achieved with a disproportionately high degree of technical effort.
When analyzing the test object, the conventional network analyzer thus quickly reaches its limits, since, at comparatively low and high frequencies, the reference signal and the measuring signal can only be decoupled with a particularly low level of quality due to a particularly marked attenuation in the excitation signal or the returning signal during decoupling on the measuring bridge. As a result, for example, the percentage of the excitation signal reflected at the test object or the excitation signal transmitted through the test object can be only inaccurately determined at high or low frequencies. The dynamics of a network analyzer of this type, which are to be understood as the ratio of the minimum and maximum measurable transmissions, are therefore comparatively low at high and low frequencies.